2022 Volume 63 Issue 1 Pages 63-68
Recent developments in wear-resistant coating technology offer new opportunities to enhance the service life of cutting tools. The wear properties of self-fluxing Ni–Cr–Mo–Si–B (Ni60A) and Ni60A–AlMgB14 composite coating fabricated by Plasma Spraying were investigated by a ball-on-disc wear tester. The results showed that the average friction coefficient of 5 mass% and 10 mass% AlMgB14 coating were 0.296 and 0.279, respectively. Compared to that of Ni60A coating, these two values decreased approximately 31.5% and 35.4%. The coatings were examined after the tests using SEM and EDS for evidence of wear mechanism and tribo-chemical reactions. The worn surface topographies of coatings indicated that the dominant wear mechanisms were abrasive wear. The low-friction behavior of the Ni60A–AlMgB14 composite coatings was due to the formation of lubricious boric acid.
The machining of hard metal with geometrically defined cutting tools is a process of considerable industrial importance.1,2) Sustained advanced regarding surface integrity, feed speed, productivity, and environmental concerns had put forward higher requirements for service life of cutting tools.3–6) As the executive components of machining, cutting tools in the ambient of oil fields or dry cutting conditions are subject to adhesion, abrasion, oxidation, diffusion and fatigue.7) Nevertheless, tool flank wear is one of the major failure forms, and that is an important limiting factor for the volume of metal removed and machining efficiency.8,9) As the beginning of the machining chain, band sawing can offer an advantage of high automation possibilities, low kerf loss, straightness of cut, good surface finish and long tool life. Therefore, minimizing the wear rate of band saw blade is a key objective for the extension of service life in the manufacture.
Significant efforts had focused on improving hardness, toughness, chemical compatibility for wear-resistant coatings of rotating or sliding tools used in industrial applications. Among the wear-resistant coatings available on literatures4,8,10–14) titanium-based hard thin films were most commonly used. The objectives of those researches tended to improve hardness of tool flank in order to reduce friction, adhesion, and chemical inertness of the contact surfaces. However, one of the shortfalls of those hard coatings is the poor lubricity, i.e., the friction coefficient (COF) is always high. Thus, it is important to reduce the friction coefficient of coating on the basis of ensuring wear resistance.
From several of literatures,7,15–22) thermal spraying or laser cladding technology Ni60A-based coating possessed excellent wear resistance, temperature resistance and oxidation resistance properties. Similarly, the high friction coefficient perplexes related researchers. Aiming to reduce the friction coefficient, tribological properties of the Ni60A/MoS2 composite coating were studied.23,24) The researches indicated that the friction coefficient of coatings was reduced sharply. The hard material AlMgB14(BAM) was first synthesized and investigated in 1970,25) and the bulk BAM materials have been intensively studied during last decade due to their high hardness, low friction coefficient and chemical inertness.26,27) Previous tribological researches indicated that AlMgB14-based coatings showed ultralow friction coefficient because of the formation of a layered-crystal structure of H3BO3 film on the sliding surface under both dry and liquid environment,28–30) and that make them be considered as an promising candidate for the use as cutting tool, cylinder and piston.30)
As mentioned above, the coating of cutting tools, such as bi-metal band-saw blade, turning or milling, are required not only wear-resistant but also low friction coefficient under the condition of dry or moisture. Accordingly, the aim of the current study was to investigate the tribological behaviors of a wear-resistant coating composited self-fluxing Ni–Cr–Mo–Si–B (Ni60A) AlMgB14 which was a promising candidate for the use as cutting tools with low friction coefficient by plasma spraying. In addition, the wear mechanism was also discussed.
The goal of this experimental work was to investigate the frictional behavior of composite powder with different proportions. Three types of powder were prepared for this study, Table 1. The particles of Ni60A powder (Chengdu Huayin Powder Technology Co., Ltd.) were spherical and less than 200 µm size. The nominal chemical composition of stellite self-fluxing Ni60A alloy powder was listed, Table 2. The BAM powder (MAT-CN Co., Ltd., 99.9% purity) was analyzed by X-ray diffraction (XRD) spectrum, Fig. 1. The XRD pattern illustrates that the BAM powder contains a large amount of Al, part of MgAl2O4 and a small amount of Si. That is because of the preparation process of BAM powder. In order to ensure that B has enough contact surfaces with Al and Mg, Al and Mg were overdose.31) The powders (No. 1 and 2) were milled using a planetary ball mill (PBM) with the ball to powder ratio (BPR) of 10:1 and rotational speed of 200 RPM. In addition, 4% polyvinyl alcohol solution (PVA) as the adhesive was uniformly pasted on the substrate surface. The milling process was carried out for 300 min. After PBM process, the composite powders were vacuumed for testing. Stereoscopic microscope images of Ni60A powder, No. 1 and 2 composite powders presented the particle size of Ni60A decreased and mechanical adhesion appeared, Fig. 2.
XRD patterns of BAM powder.
Stereoscopic microscope images of Ni60A powder (a), Ni60A–5 mass% AlMgB14 (b) and Ni60A–10 mass% AlMgB14 (c) powders after PBM process.
M42 high speed steel was used as a substrate material for the plasma spraying process and machined to gage size of 20 mm diameter × 1.1 mm thick. The specimens had been polished in steps before sandblasting process. The specimens were sand blasted at the pressure of 0.5 MPa with corundum, and then ultrasonically cleaned in acetone, ethanol, and deionized water successively for 15 min, and finally dried in air. The obtained Ra of the substrates was between 7 µm and 13 µm.
2.3 Preparation of the coating sampleThe plasma spraying process was carried out using a Praxair-Tafa 3710 plasma spray system (Praxair Surface Technology, USA) with a SG-100 spray gun mounted onto a Motoman Robot for manipulation consistency. The parameters of plasma spraying are listed, Table 3.
The self-fluxing alloy powder containing B and (or) Si elements and thus have a low melting point of about 950–1150°C. In addition, the self-fluxing alloy powder is a kind of alloy with deoxidation, slagging, degassing and good wetting properties, and has the function of self-deoxidation and slagging. Therefore, the moderate temperature for thermal spray can also effectively reduce the decomposition of the carbide/boride ceramic reinforcement phase and maintain its good mechanical properties such as high hardness and mechanical strength. For that reason, the process of heated or self-fluxing had not been carried out in this test.
2.4 Characterization of coatingThe morphology of the coatings and worn surface were examined using scanning electron microscopy (SEM, EVO-18, ZEISS, German). The SEM images were obtained using back-scattered electrons at operating voltage of 20 kV. The phase composition of the coatings was identified using an X-ray diffractometer (XRD, XRD-6100, Shimadzu, Japan) operated with CuKα radiation (λ = 0.1541 nm). The ball-on-disc wear tests were performed on an UMT tester (CETR Ltd., USA). This tester can produce synchronized combinations of linear and rotary motions with in situ measurements of tribological parameters, including friction forces and coefficient, wear depth, contact acoustic emission, etc. Wear tests were performed with Si3N4 ball (Diameter 6 mm) sliding over 5 mm at 60 mm/s with 100 N normal load (Fz) in reciprocating mode using a linear drive. The ball was mounted under the sensor using a force suspension and the diameter. Friction coefficient and Fz forces were recorded during all tests. Wear test was conducted for 300 s (about 18000 mm). Energy dispersive spectroscopy (EDS) was used to analyze the composition of the worn surface.
From the surface morphologies of the coating of S1, it can be seen that the surface of the coatings was uneven and has a certain degree of roughness, Fig. 3. A certain amount of micro-cracks and non-molten powders were observed. The phenomenon of cracks resulting brittle fracture for top coating is mainly attributed to thermal stress. When the heated particles contained bubbles, it will disperse when it hits the substrate surface, forming thin-layer particles. Under the action of thermal stress, thin-layer particles will produce cracks on the surface of coating. The presence of the non-molten nanoparticles in the sprayed powders was mainly related to the spraying parameters affecting the dwell time of the powders experienced during plasma spraying and the temperature of the powders. The high velocity and temperature gradient from the outside to the inside of the agglomerated powders in the plume help to preserve the nanostructure inside of the powders.32) The X-ray diffraction spectrum of coatings showed the main phase of coatings were mainly composed of Ni3Fe, SiC, AlCrFe2 and Cr23C6, Fig. 4. The coating was formed by these phases, and helps to increase wear resistance.
SEM morphologies of the surface of S1 (Ni60A–5 mass% AlMgB14).
XRD pattern of the coatings of S0 (Ni60A), S1 (Ni60A–5 mass% AlMgB14) and S2 (Ni60A–10 mass% AlMgB14) after plasma spraying process.
From the typical curve of the friction coefficient vs. sliding time for samples at the applied load of 100 N in dry medium and constant sliding speed of 60 mm·s−1, Fig. 5, it is can be seen that the fluctuation of friction coefficients corresponded to the variation of sliding surface state resulted from uneven surface, Fig. 3. In the running-in period or at the early stage, the frictional interactions took place at discrete micro-contact areas, which were the muti-asperities on the coating surface. From the wear tests, muti-asperities of S1 and S2 coating were easily plastically deformed under the harder material (Si3N4 ball), leading to a longer running-in period of the two samples. Furthermore, with the increase of BAM powder, the running-in period also increases accordingly. As observed, the friction coefficient reaches the stable stage after an initial running-in time, and that was because of the decrease of surface roughness. In the stable stage, the COFs of S1 and S2 were much lower than that of S0, and the composite powders decreased the COF of coating. The hardness of abrasive particles of S0 is larger than the other two samples, which leads to an increase in the vibration amplitude of Si3N4 ball of UMT tester. Therefore, Owing to the BAM, the frictional wear stability of S1 and S2 superior to S0. The average friction coefficient of samples tested against Si3N4 balls is presented, Fig. 6. From the histogram, it can be seen that the average friction coefficient of S0 (about 0.432) was much higher than S1 (about 0.296) and S2 (about 0.279). Compared to S0, the values of average friction coefficients of S1 and S2 decreased approximately 31.5% and 35.4%, respectively. The S1 and S2 had approximate value close to 0.17. From the error bar we can also infer that the addition of BAM increased the friction stability of the coating.
Time dependence of coefficient of friction (COF) for S0 (a), S1 (b) and S2 (c) tested against Si3N4 ball at 100 N load in dry medium at a constant sliding speed of 60 mm·s−1.
The average friction coefficients for S0, S1 and S2 tested against Si3N4 ball at 100 N load in dry medium at a constant sliding speed of 60 mm·s−1.
Under the same wear condition, the higher value of average wear depth, the higher the wear.30) The average wear depth of S0 (about 0.1262 mm) was slightly higher than S2 (about 0.1216 mm), and much higher than S1 (about 0.0908 mm), Fig. 7. Compared to S0, the values of average wear depth of S1 and S2 decreased approximately 28.1% and 3.65%, respectively.
The values of average wear depth for S0, S1 and S2 tested against Si3N4 ball at 100 N load in dry medium at a constant sliding speed of 60 mm·s−1.
The worn surfaces of the three samples under 100 N load in dry medium at a constant sliding speed of 60 mm·s−1. Those indicate that the dominant wear mechanisms were ploughing, Fig. 8. The surfaces exhibit a number of scratches, cracks and wear particles most probably produced as a result of abrasive wear. In the early wear stage, the contact state of the friction pairs was the asperity contact between the coating and the Si3N4 ball. With the action of the stress, the wear particles and wear debris broke loose as a wear particles, and new particles and debris appeared, repeatedly.
Morphologies of worn surfaces of S0 (a), S1 (b) and S2 (c).
Compared with Fig. 8(b) and (c), Fig. 8(a) exhibits more severe wear. Many parallel wear scars on the worn surface can be observed, Fig. 8(a). The strip worn marks (such as scratches) and lamellar exfoliation (such as wear particles) were presented on the worn surface. In addition, cracks also appear on the worn surface of the S0, S1 and S2. The phenomenon of cracks resulting from brittle fracture for coating is mainly attributed to the tangential force during the sliding friction process. The asperities were cut off under the shear force, and then the spalled coating squeezed the surface of the coating to form cracks.
The representative wear particles of the three samples were marked as 1, 2 and 3 (Observed by the Fig. 9(a), (c) and (e)), respectively. It can see that the chemical compositions of worn particles of Ni60A coating consist of a large amount of Ni, C and O, Fig. 9(b). The results of EDS demonstrate that the main components of the particles and debris were Ni3Fe, AlCrFe2, Cr23C6 and delamination of oxide film, and that demonstrates that the oxidation wear occurred during the whole wear process. It can be seen that the main chemical composition of worn particles of Ni60A–AlMgB14 was B as shown in Fig. 9(d) and (f). Based the literature,1,29,34,35) the low-friction mechanism for the AlMgB14-based coatings is believed to be due to the formation of boron oxide, B2O3, and subsequent formation of layered boric acid, B2O3·3H2O, or B(OH)3 at the worn surface. The reaction is shown in eq. (1)
| \begin{equation} \frac{1}{2}\text{B$_{2}$O$_{3}$} + \frac{3}{2}\text{H$_{2}$O} \to \text{B(OH)$_{3}$} \end{equation} | (1) |
Morphologies of worn surfaces of S0 (a), S1 (c) and S2 (e), and the EDS analysis of worn particle 1 in (b), 2 in (d) and 3 in (f).
The dry sliding tribological properties and wear mechanisms of the Ni60A–AlMgB14 coatings were investigated. The following conclusions can be drawn:
This study was supported by the State Key Laboratory of Automotive Safety and Energy (No. KF1814), National Natural Science Foundation of China (No. 51875242 and 51475205), National Key Research Program of China (No. 2016YFD0701601 and 2017YFD0701103-1), Jilin Province Science and Technology Development Plan Item (No. 20190302129GX and 20170101173JC), Jilin Provincial Development and Reform Commission (No. 2018C044-3), Jilin Province Education Department “13th Five-Year” Industrialization Project (Grant No. JJKH20180076KJ), China-EU H2020 FabSurfWAR project (No. 2016YFE0112100 and 644971), the 111 Project of China (No. B16020).
